Indirect arc metal melting furnace

ABSTRACT

An indirect arc metal melting furnace employing an ionizable interior atmosphere is provided with a sealed and insulated double shell construction which is effective to minimize moisture outgassing from an internal crucible lining and thereby improves the electrical operating characteristics of the furnace.

United States Patent 1191 Greenewald, Jr.

INDIRECT ARC METAL MELTING FURNACE Inventor: Herbert Greenewald, Jr., 4296 Braunton Rd, Upper Arlington, Ohio 43220 Filed: Apr. 27, 1973 Appl, No.: 355,285

Related U.S. Application Data Continuation-impart of Ser. No. 151,937, June 8, 1971, abandoned, which is a continuation-in-part of Ser. No. 24,490, April 4, 1970, abandoned.

U.S. Cl 13/9, 13/31, 13/35 Int. Cl. H05h l/00, H05b 7/18 Field of Search... 13/9, 9 P, 31, 35

References Cited UNITED STATES PATENTS Stassano ct al. 13/35 1 1 June 28, 1974 Booth 1.1/35 Merrick 13/10 X Wcinheimer ct a1. 13/10 X Buck ct al 13/31 X Shaw 1 1 13/10 X Beasley et al 13/9 X Petersen et a1 .1 13/9 P Primary Examiner-Roy N. Envall, Jr. Attorney, Agent, or Firm-Jackson, Jackson and C hovanes ABSTRACT An indirect are metal melting furnace employing an ionizable interior atmosphere is provided with a sealed 'and insulated double shell construction which is effective to minimize moisture outgassing from an internal crucible lining and thereby improves the electrical operating characteristics of the furnace.

6 Claims, 4 Drawing Figures PAIENTEDJUN28 19m INVENTOR. HERBERT GREENEWALQJR Fl (5. 3

, I INDIRECT ARC METAL WLTING FURNACE CROSS REFERENCES This application is a continuation-in-part application for US. patent application Ser. No. 151,937 (now abandoned) which is in turn a continuation-in-part of Ser. No. 24,490 filed Apr. 4, 1970 (now abandoned).

SUMMARY OF THE INVENTION An indirect are metal melting furnace is provided with a sealed double shell construction containing a porous interior crucible lining that may be minimized as to thickness because of insulation incorporated in the double shell. An ionizable gas such as elemental argon, vaporized alkali metal or alkaline earth metal halide, vaporized zinc metal, vaporized magnesium metal, vaporized aluminum metal, or combinations of any of the preceding with other gases including nitrogen, carbon monoxide, metal oxide vapors, argon, helium, hydrocarbons, and hydrogen, is contained in the substantially sealed crucible interior and opposed graphite electrodes projected into the crucibleinterior are energized through water cooled and sealed electrode holders by a power source such as a constant voltage, variable current power supply. While both constant current and constant voltage transformers may be used as power supplies, the best electrical efficiency is obtained using constant voltage transformers as a power supply. While other indirect arc furnaces require the use of constant current transformers or high impedance (35 percent) transformers as a power supply. While other indirect arc furnaces require the use of constant current transformers or high impedance (35 percent) transformers with consequent large transformer power losses and low power factor, the indirect arc furnace of this invention may be operated with low impedance (4-5 percent) constant voltage transformers of the distribution type. This serves to greatly reduce the capital cost of the transformer and the electrical power consumed in the operation of the transformer system. Shell construction characteristics, like the preferred crucible constructions employed in the furnace, are such as to not induce those magnetic or electrostatic fields which during furnace operation adversely affect arc stability.

DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are elevational and plan views, respectively, of a preferred embodiment of the indirect are metal melting furnace of this invention.

FIG. 3 is a sectional view of an electrode holder construction preferred for the furnace illustrated in FIGS. 1 and 2.

FIG. 4 is a sectional view detailing a form of seal employed in the furnace construction illustrated in FIGS. 1 and 2.

DETAILED DESCRIPTION One embodiment of the indirect are metal melting furnace of this invention is references generally as in FIGS. 1 and 2 of the drawings. Basically, furnace It) is comprised of a rotatable shell assembly ll, supports 12 and 13 that rotatably support assembly Ill through the water cooled trunnions 14 and 15 welded to the outer and inner shells l6 and I7, and an energy source such as a constant voltage, variable alternating current electrical power supply (illustrated schematically as 118) electrically connected to the slip terminals 19 that cooperate with water cooled electrode holders 20. Trunnions I4 and I5 each include a horizontal baffle (not shown) generally along the axis of rotation to effect complete cooling by water flowed through the trunnion interior from bottom inlet hose 21 to upper outlet hose 22. This water cooling is needed primarily to remove heat developed in the trunnion metal by eddy current, induced current flow, and hysteresis losses occasioned by the magnetic fields generated by the flow of current through the electrode holders. It is therefore necessary and possible to minimize power losses to the cooling water or eliminate the need for cooling water by increasing the distance between the outside diameter of the electrode holder or electrode proper and the inside diameter of the metal trunnion wall. In practice it has been found that this spacing should be held to a minimum of about 2 inches if heating of the trunnion wall is to be held to reasonable levels. Trunnion l5 differs from trunnion l4 essentially only in that it is provided with a drive sprocket 23 for effecting shell assembly rotation. Any conventional powered drive may be used for turning assembly ll through sprocket 23.

Outer and inner shells l6 and I7 are preferably made of mild steel and are provided with circular bottom plates 24 and 25 and with an annular top plage 26, also of mild steel, welded thereto. Bottom plate 24 in the illustrated furnace embodiment is provided with an inlet fitting 26' which cooperates with an argon supply hose 27. Gas injection need not necessarily be through a porous plug and crucible bottom in that the gas may be introduced from the crucible top or other crucible zones or through the electrode holders if suitable provi- Sions are made. Plate 25 is supported by annular pipe spacer 2S and is provided with drilled holes 29 in communication with the porous ceramic material 30 located interiorly ofspacer 28 and with inlet fitting 26. The components of shell assembly may be welded together but it is only required that the inner shell be reasonably gas tight. The outer shell may be attached by welding or by brackets bolted to the inner shell. The insulation between inner and outer shells may be rigid, fibrous, or of the radiation shield type. The only requirement is that the inner shell be insulated so that it readily attains in operation a temperature sufficient to completely outgas the refractory materials contained within the inner shell. If conventional refractories are used this may require the inner shell to be heated throughout to temperatures of about 1,750F. and in this case it is best that the'inner and outer shells be welded together and the space between them be filled with rigid and structurally supportive refractory materials. If newly developed refractory materials which completely outgas at temperatures of 800F. and below are used this inner shell temperature may be limited to 800F., and in this case the simple encasement of the inner shell with insulating blankets, radiation shields, or other such insulating means without the necessity of the outer shell providing structural support for the inner shell is adequate. Insulating material is packed into the zones intermediate shells l6 and 117 and intermediate plates 24 and 25 before the assembly is closed by the attachment of plates 24 and 26 into proper position. If the inner and outer shells are welded together,

appropriate expansion joints must be provided in the inner shell.

The interior of shell assembly 11 is provided with a crucible lining 31, which may be constructed of premolded refractory crucible halves, brick, or cast or rammed refractory in monolithic form. In the furnace embodiment shown the lower crucible half rests on plate 25 and is surrounded by a backup refractory filler 32. Openings are provided in the crucible 31 halves in registry with the interior openings of trunnions 14 and 15 to facilitate insertion and withdrawal of electrodes 33. Electrodes 33 may be made of a graphite material and of other materials such as tungsten.

As an alternate form, instead of two electrodes properly placed and equipped, the furnace may be provided with three electrodes suitably connected as to threephase current or more than three electrodes.

The diameter of these electrodes is important to the successful operation of the furnace and is determined by the current flowing through them. The attachment of the graphite electrodes to a watercooled electrode holder permits the electrode to be heated to a red. heat or above throughout most of its length by the electrical resistance of the electrode material to the passage of current while keeping the portion of the electrode in immediate contact with the holder relatively cool. For example, it is feasible to have an electrode with three clearly observable temperature zones in operation in the furnace. The zone of the electrode in immediate contact with the holder and for about 6 inches out from the holder is in the black heat range, the very tip of the electrode emitting electric current into the arc is white hot, and the intermediate zone is a bright red or yellow heat depending on the furnace temperature. It is important to the operation of the furnace that the relationship of electrode diameter to current (not power but just amperes of current) flowing through the electrodes be such that the whole tip of the electrode may be kept at about a white heat and that stable emission of electricity into the arc zone without erratic movement of an arc spot be maintained. For example, for a current flow of 2,000 amperes a 2 inch diameter graphite electrode is suitable. In a conventional indirect arc furnace an electrode diameter of to 6 inches would be.used for this current flow. Obviously, the graphite electrode must be protected from oxidation throughout its hot zone if it is to remain in use for any appreciable length of time. On the other hand, if too small a diameter electrode is used it will be resistance heated to a temperature which will cause the refractory in the electrode hole and particularly at the intersection of the electrode hole with the interior of the crucible to melt. A closable discharge opening 34 is provided in the upper crucible half as well as an inlet opening 35 that cooperates with the longitudinal interior lining opening 36 (FIG. 2) in charging chute 37. By way of example, an inverted relatively impermeable ceramic casting mold or a metal ingot mold may be placed with its ingate overopening 34 and placed abutting to top plate 42 to serve also as a closure member. Alternatively a closure flange or plate may be provided. A flange 38 is preferably welded in sealed relation to the free end of chute 37 and is provided with a groove 40 for receiving a conventional O-ring seal. During operation of furnace apparatus a heavy clear acrylic plate may be clamped to the face of flange 38 to provide a sealed viewing port. In other embodiments of furnace apparatus incorporating the present invention it may be preferred to utilize the conventional valved charging lock at the free end of chute 37 rather than a viewing port type of opening to the crucible interior. It may also be preferred to provide both a charging and a viewing port. The crucible 31 is held in position in shell assembly 11 by a circular mild steel top plate 41 secured to annular top plate 26 by temporary welded clips 42 or by bolted attachments. Threaded rods 43 welded to outer shell 16 are provided for use in conventionally clamping an inverter mold on top of assembly 11 with its feed gate aligned with opening 44 in plate 41 and with opening 34 in crucible 31.

Each electrode holder 20 preferably cooperates with a ceramic sleeve 45 positioned interiorly of .a water cooled trunnion and additionally cooperates with a conventional elastomer (e.g. molythane) cup and O- ring seal 46 which provides both sealing against the passage of gas and electrical insulation between the electrode holder and the trunnion outermost end. See FIG. 4. A preferred construction for each electrode holder 20 in order that it might be adequately cooled by water flowed to and from inlet and outlet water hoses 47 and 48 is illustrated in detail in FIG. 3. As shown in that illustration, electrode holder 20 is essentially made up of threaded copper end fitting 50 engaged with graphite electrode 33, end fitting 51 having inlet hose fitting 52 and inlet tube 53 securely attached thereto, and heavy walled exterior tube 54 welded to end fittings as shown. An outlet hose fitting 55 cooperates with tube 54 near end fitting 51 and normally is located in a top position for cooperation with outlet hose 48 (FIG. 1). From the drawing it will be noted that cooling water is ported through fitting 52 and tube 53 to first play against end fitting 50 and then counterflows through the zone between tubes 53 and 54 for discharge through fitting 55 thus effecting electrode holder and electrode attach end cooling in a very efficient manner. The indirect arc metal melting furnace embodiment illustrated in the drawings is rotatable and utilizes a single pair of opposed electrodes. It will be readily appreciated that alternate embodiments might be provided utilizing the same basic principles of shell and crucible construction but having multiple pairs of opposed electrodes or having a rotational axis positioned near the the furnace shell metal discharge opening. Although the FIGS. 1 and 2 apparatus embodiment is normally constructed so that the axes of electrode holders 20 and coincident axis pass through the appara tus center of gravity, such is not necessary or even desirable in all metal melting furnace constructions.

The key features of the indirect arc furnace of this invention are:

1. Provision of a double shell arrangement which permits the heating of all of the refractory contained within the inner shell to a temperature at which all water, combined chemically or otherwise, is eliminated.

This is necessary if a truly controlled atmosphere com- 3. Provision of at least one pair of graphite or tungsten electrodes projecting into the furnace cavity and connected to an exterior source of electric power, through attached and water cooled electrode holders. These electrode holders enter the furnace through gas tight electrically insulating seals.

4. The electrodes are sized so that most of their length is heated to a red heat or above by their resistance to the flow of electric current and the arcing tip of the electrode is uniformly heated to an approximate white heat during arc operation.

The major differences in furnace operating characteristics achieved in this invention as compared to other indirect arc furnaces are:

1. The achieved power factor in the electrical operation of the furnace without the need for external power factor correction devices is approximately l as compared with power factors of from one-third to one-half in conventional indirect arc furnaces.

'2. Arc lengths for any given voltages are greatly increased. For example, in a normal indirect arc furnace at an arc voltage of AC 44 volts the arc gap would be about 1/16 inch and in this invention would be about Me inch. At 200 volts the respective arc gaps would be about A inch and inches. At 400 volts the respective arc gaps would be /2 inch and inches.

3; Because of the much longer arcs achieved in this invention a more uniform distribution of heat throughout the furnace is achieved. In particular the shadow effect of the electrode ends which is quite significant in the Detroit Rocking Arc Furnace becomes totally insignificant in the furnace of this invention.

4. In this furnace the flow of electricity through the arc at constant voltage can be easily controlled by varying the arc length by varying the electrode spacing. Since thefeasible arc lengths are quite long this adjustment is possible to achieve over a wide range of current flows. Specifically, power input can be adjusted over a ratio of about 110111 with a constant voltage power supply simply by varying the length of the arc gap. This is totally impossible with conventional arcs because the feasible range of arc lengths for a given applied voltage is too short to give any significant current flow adjustment by means of arc gap change. The usual method for varying power input to an arc furnace is by changing the applied voltage through tap transformers and other such expensive mechanisms.

5. The furnace of this invention, as a result of the ability to control the composition of the internal atmosphere, is able to melt steels,. coppers, nickel base superalloys and obtain technical results identical to those obtainable in vacuum induction melting without the need to resort to the use of expensive vacuum equipment.

6. Another unexpected and unusual effect of the use of this invention is the very substantial reduction in the amount of graphite consumed through electrode wear.

It is generally recognized that in conventional arc furtions. For example, for a power input of 200 kw the graphite consumption is reduced from a rate of about 4 inches per hour of electrode in a conventional furnace to about /1. inch per hour in the furnace of this invention. Under some operating conditions the rate of graphite consumption in the furnace of this invention is as little as Vs inch per hour at 200 kw power input.

It is important in providing metal melting furnace apparatus incorporating the instant invention that the crucible 3ll and refractory lining 32 contained within inner shell 17 not induce magnetic fields which during furnace operation would adversely affect otherwise obtainable furnace electrical performance characteristics. If crucible 31 is constructed entirely of an electrically conductive refractory material such as graphite, for instance, the induced magnetic fields resulting from furnace operation with an alternating current power supply apparently cause the are established by electrodes 33 to be and remain unstable. If the construction of crucible 311 is modified, on the other hand, from an allgraphite or other equivalent refractory composition to one in which upper and lower conductive crucible portions are separated by an electrically insulating annular crucible section, such as a section made of alumina, magnesia, zirconia, or a like refractory, a stable arc is obtained apparently because the electrical current patterns set up in the upper and lower crucible portions do not combine to form an induced magnetic field adversely affecting arc stability.

Some electrically conductive refractories such as graphite have high strengths and great resistance to thermal shock at elevated temperatures and accordingly may advantageously be incorporated into furnace apparatus interiors continuously utilizing a nonoxidizing atmosphere. Specifically, in instances in which metals not reacting with or dissolving carbon significantly, such as copper or aluminum, are to be melted in the furnace 10, a crucible can be constructed with a graphite interior roof or with a graphite interior roof and or with a graphite interior roof and interior floor set into a non-conductive crucible refractory lining. Arc stability can be maintained continuously provided the requisite degree of ionization is maintained in the crucible interior atmosphere. Also, it should be noted that the construction of furnace shell assembly 11 is such that the various mild steel top and bottom plates 24 through 26 and 4-1 and inner and outer shells l6 and 17 do not serve to induce adverse magnetic fields during furnace operation when properly placed in the normal illustrated positions.

In order that the metal melting furnace of this invention might operate in the desired manner, it is also necessary that an ionizable gaseous atmosphere such as vaporized alkali metal halide or alkaline earth halide or a reasonably pure argon atmosphere be maintained in the crucible interior during the operation of power supply 18 to conduct electrical current between electrodes 33. Two halides found satisfactory in particular applications for this purpose are potassium chloride and calcium fluoride charged in proportionally small amounts. Failure to maintain such atmosphere will normally result in rapid electrode consumption, in a low power factor causing a low power input and generally inefficient furnace operation, and in highly undesirable electric arc instability. Undesirable moisture impurity additions to the atmosphere provided to the furnace interior may be sourced in the leakage of atmospheric air into the furnace crucible interior or in the outgassing of moisture (or even otherchemical impurities) from the crucible refractory lining. Such outgassed moisture may be sourced either as occluded moisture at the crucible interior surface or as water of crystallization chemically combined into the crucible composition being utilized and released by exposure to the high temperatures required in the furnace interior to melt metal.

The problem of minimizing air leakage into the furnace crucible interior is also conveniently controlled by known electrode sealing techniques such as are shown in the drawings. Moisture outgassing, on the other hand, apparently is satisfactorily controlled in either one or both of two manners.

First, and not a part of this invention, moisture outgassing may be controlled in part by the selection of crucible lining compositions which do not contain combined water of crystallization. Cured castable (not containing hydraulic cements) or rammable refractory comprised of essentially pure alumina, for instance, meets this objective and is known to be at least under development if not already available. Compositions disclosed in U.S. Pat. No. 3,164,872 are also suitable. Absent a composition that is free of combined moisture or water of crystallization, a conventional castable or rammable refractory composition must be used in a crucible and must typically be fired to a temperature of 1,500F. to 2,000F. throughout to effect complete moisture separation and removal from the crucible interior. Occluded water, on the other hand, may be completely removed from the crucible upon heating the interior of shell 17 throughout to temperatures substantially above the associated boiling point temperature. Such water normally comprises a very small fractional part of the contained water in a complete furnace systems having crucibles of conventional refractory composition.

The other manner of controlling moisture outgassing in a metal melting furnace system involves utilization of the furnace construction shown in the drawings. Basically, apparatus 10 is constructed so that inner shell 17 is located at about a 1,400F. position on the furnace thermal gradient existing between the interior of crucible 31 and the exterior surface of shell 16 during normal continuous operation. By utilizing the disclosed double shell construction for assembly 11, the refractory line 32 as well as the crucible component 31 more quickly completely reach a temperature during furnace steady operation whereat the contained water is entirely driven off and the electrical arc becomes stabilized. By way of example, a conventionally constructed melting furnace with single furnace shell and having a 200 pound metal melt capacity, requires approximately three weeks of continuous heating under vacuum to completely remove contained water from a conventional refractory lining and arrive at a stabilized arc operating condition. The same furnace operating under identical conditions except that a refractory liner material containing no water of crystallization was employed, required approximately four hours operation under vacuum conditions to completely eliminate all contained moisture and stabilize arc operation. By the further use of a double shell construction as shown in the drawings to achieve a minimum refractory lining thickness, the time required to completely outgas the water of crystallization from the refractory without the need for vacuum conditions was reduced by approximately 50 percent to 2 hours for complete water removal. An improved operating power factor is more rapidly realized and concomitant reduction in rate of electrode consumption from approximately 4 inches per hour to 5 1 inch per hour was also achieved sooner by improving arc stability. A requirement for continuous adjustment of electrode position for nonintermittent electrode operation was quickly reduced to adjustment-free electrode operation for periods of from 15 minutes to 30 minutes. Also, since the regions intermediate furnace shells l6 and 17 are sealed, scaling of the metal walls by oxidation attack is minimized and furnace life significantly prolonged.

Two comments are in order with respect to the electrical characteristics of apparatus 10 when an altemating current electrical supply is employed for furnace operation. First, an apparent power factor of l percent) is developed with a proper atmosphere and no adverse induced magnetic fields. Second, the atmosphere breakdown voltage on each half cycle under the same operating conditions is low compared to the peak voltage and is relatively constant. Higher operating voltages therefore effect greater electrical efficiencies.

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art to obtain all or part of the benefits of my invention without copying the method and apparatus shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Having thus described my invention what I claim as new and desire to secure by letters patent:

1. In indirect arc electrical metal melting apparatus, in combination:

a. outer metal shell means having electrode openings;

b. inner metal shell means spaced apart inwardly from the outer metal shell means, having electrode openings aligned with said outer metal shell means electrode openings, and joined to said outer metal shell means to define a substantially sealed and supportive intermediate insulation zone;

0. insulation material contained in said sealed intermediate zone substantially coextensive with said inner and outer metal shell means;

d. crucible means constructed of a refractory composition, contained within and supported by said intermediate shell means, and having a metal conducting opening and electrode openings located above the molten metal melt top surface level and aligned within said inner and outer metal shell means electrode openings, and

. linearally adjustable electrode means operatively connected to an energy source and projected into the interior of said crucible means through said crucible means electrode openings and said inner and outer metal shell means electrode openings, and in sealed and electrically insulated relationship to said inner and outer metal shell means, said inner metal shell means being located at a temperature position on the thermal gradient existing between the temperature of the molten metal contacting surface of said crucible means and the temperature of the exterior surface of said outer metal shell means, each during operation of the apparatus for metal melting purposes, that is above the temperature of complete preparation of water of crystallization from said crucible means refractory composition.

2. The invention defined by claim 1, wherein said inner metal shell means is located at a temperature position in the range of approximately from l,500F. to 2,000F. on said thermal gradient.

3. The invention defined by claim 11, wherein said insulation material is rigid insulating refractory operating in combination with expansion joints in said inner metal shell means to overcome the tendency of said inner metal shell means to thermally expand away from the contained crucible.

4. The invention defined by claim ll, wherein said crucible means is constructed to have an electrically conductive refractory composition top portion and an electrically conductive refractory composition bottom portion, said crucible means top and bottom portions being separated from mutual contact by electrically non-conductive refractory composition.

5. The invention defined by claim 4, wherein said crucible means top and bottom portions are comprised of graphite.

6. The invention defined in claim 1, wherein said crucible means is provided at its interior with an ionizable atmosphere selected from the gas group comprised of argon, the vaporized halide of alkaline metal halides and alkaline earth metal halides, zinc metal, magnesium metal and aluminum metal. 

1. In indirect arc electrical metal melting apparatus, in combination: a. outer metal shell means having electrode openings; b. inner metal shell means spaced apart inwardly from the outer metal shell means, having electrode openings aligned with said outer metal shell means electrode openings, and joined to said outer metal shell means to define a substantially sealed and supportive intermediate insulation zone; c. insulation material contained in said sealed intermediate zone substantially coextensive with said inner and outer metal shell means; d. crucible means constructed of a refractory composition, contained withiN and supported by said intermediate shell means, and having a metal conducting opening and electrode openings located above the molten metal melt top surface level and aligned within said inner and outer metal shell means electrode openings, and e. linearally adjustable electrode means operatively connected to an energy source and projected into the interior of said crucible means through said crucible means electrode openings and said inner and outer metal shell means electrode openings, and in sealed and electrically insulated relationship to said inner and outer metal shell means, said inner metal shell means being located at a temperature position on the thermal gradient existing between the temperature of the molten metal contacting surface of said crucible means and the temperature of the exterior surface of said outer metal shell means, each during operation of the apparatus for metal melting purposes, that is above the temperature of complete preparation of water of crystallization from said crucible means refractory composition.
 2. The invention defined by claim 1, wherein said inner metal shell means is located at a temperature position in the range of approximately from 1,500*F. to 2,000*F. on said thermal gradient.
 3. The invention defined by claim 1, wherein said insulation material is rigid insulating refractory operating in combination with expansion joints in said inner metal shell means to overcome the tendency of said inner metal shell means to thermally expand away from the contained crucible.
 4. The invention defined by claim 1, wherein said crucible means is constructed to have an electrically conductive refractory composition top portion and an electrically conductive refractory composition bottom portion, said crucible means top and bottom portions being separated from mutual contact by electrically non-conductive refractory composition.
 5. The invention defined by claim 4, wherein said crucible means top and bottom portions are comprised of graphite.
 6. The invention defined in claim 1, wherein said crucible means is provided at its interior with an ionizable atmosphere selected from the gas group comprised of argon, the vaporized halide of alkaline metal halides and alkaline earth metal halides, zinc metal, magnesium metal and aluminum metal. 